When an electric motor is started, the electric current drawn by the motor can be six times the steady state current once it reaches full speed. Manufacturing equipment and assembly lines often have a number of relatively large three-phase electric motors which start simultaneously thereby placing very large current demands on the electrical distribution system feeding the equipment or assembly line.
In order to reduce this start-up current consumption, large alternating current electric motors are often operated by a controller. When the motor is to be started, the equipment operator applies a starting signal to the motor controller. As is well-known, the motor controller then gradually increases the amount of current applied to the motor by regulating the duty cycles of thyristors coupling each phase of electricity to the motor. In doing so, the controller turns on the thyristors initially for only a brief portion of each half-cycle of the A.C. voltage for the corresponding electricity phase. The controller then gradually increases the half-cycle on time of the thyristors until they are constantly turned on at which time the motor is at substantially full speed. This technique reduces the current consumption and torque of the motor during start-up as compared to a hard switching of the full supply line voltage across the motor.
Previous motor controllers often did not provide a mechanism for braking the motor when it was stopped. In response to an operator input to stop the motor, the basic controller simply turns off the thyristors allowing the motor to coast to a stop, slowed only by friction. If the motor is coupled to a mechanical load with considerable inertia, the motor and the load will continue to move for some time after the power is shut off. In many industrial applications of motors, it is important for convenience and efficient use of the driven equipment to stop this continued movement as fast as possible. Merely allowing the motor to coast to a stop often is unsatisfactory. Heretofore, a mechanical brake frequently was coupled to the equipment and engaged when the power was turned off.
As an alternative, a direct current was sometimes applied to the stator windings of an alternating current motor to provide a braking action. In order to electrically brake an alternating current motor, it is necessary to generate a torque in the direction opposite to the direction of the rotation of its rotor. In the direct current braking method of the prior art, the torque is produced by the rotor attempting to rotate in the presence of a steady magnetic field produced by the direct current applied through the stator windings. The rotating direction of the rotor's magnetization leads the direction of the magnetic field produced by the direct current through stator winding. The tendency of the rotor magnetization to align itself with the stator's magnetic field creates an alignment torque which produces a braking effect on the rotor. As is well-known, this torque is equal to the product of the stator magnetic field strength and the rotor magnetization together with the sine of the angle between the two.
Another method of braking the motor involves switching the alternating current to the motor at the proper times to create a magnetic field within the motor which tends to slow the rotor. This technique is described in U.S. Pat. application Ser. No. 103,729 filed on Oct. 2, 1987 and assigned to the same assignee as the present invention.
One of the problems inherent in any braking technique that applies electricity to the motor, is determining when the motor has stopped so that the application of the braking electricity can be discontinued. Not only is the continued application of the braking electricity inefficient from an energy conservation standpoint, but it may also have adverse effects on the motor.
Heretofore, a timer was often employed for such braking methods with the braking electricity applied for a long enough interval to insure that the motor was stopped. This interval had to be empirically set by the operator for each specific braking application. If the load on the motor varies, thereby affecting the braking time, the interval would have to be set for the worst case, or longest braking interval. This too would be inefficient when the load inertia was small and the motor stopped in a fraction of the worst case interval.